1. Field of the Invention
The present invention relates to a method and apparatus for withdrawing supernatant from a reactor without drawing floating solids or scum during a decanting operation. The present apparatus and method are particularly useful in biological wastewater treatment operations, such as a sequencing batch reactor (SBR), an aerobic digester, or the like.
2. Brief Description of the Prior Art
A number of decanting systems have been developed to withdraw supernatant from a basin following the settling phase in an SBR. These devices can generally be classified as a fixed type decanter or a floating type decanter. The object of these devices is to withdraw the supernatant without withdrawing floating solids or scum during the decant sequence. Equally important is that the suspended solids are also prevented from entering the decanter receptacle during non-decant sequences.
An example of a floating-type decanter is disclosed in U.S. Pat. No. 4,695,376 to Astrom et al. This floating decanter attempts to prevent floating solids or scum from entering the stream of decanted supernatant via a weir-to-float interface. During non-decant sequences, the weir is held against the underside of the float (in a submerged position) in a sealed arrangement for preventing suspended solids from entering the weir. After the settling phase of the SBR, a linear actuator is utilized to lower the weir from the underside of the float, thereby enabling the weir to decant the supernatant.
Unfortunately, during the fill, react, and settling SBR phases, suspended solids enter and accumulate in the decanter receptacle due to the inability of the sealing arrangement to provide an absolute seal. Consequently, during the initial decanting operation, a higher solids content is evident in the decanted supernatant. In many instances, the solids content necessitates that the system be flushed. Typically, the system is flushed by recirculating the first several minutes of decant fluid. This operation flushes the lines of solids settled in the decanter during non-decant sequences, but does not ensure effluent suspended solids consistent with the quality of the clarified supernatant.
One proposed solution to these problems is described in U.S. Pat. No. 5,104,528 to Christie. The Christie patent discloses a floating decanter apparatus, which includes a weir for decanting supernatant and a float for buoyantly supporting the weir in the basin. The Christie patent discloses a single, centrally-located actuator to raise and lower the weir. This configuration allows the weir to tilt and may result in poorly-controlled discharge rates, as well as discharge containing suspended solids. The Christie patent further discloses a valve at the basin outlet end of the effluent pipe to retain water inside the weir when it is shut down. The valve is necessary because the weight of the decanter is insufficient to push the empty (buoyant) weir under the water. The floatation ring is lifted out of the water when the weir is empty. It is then necessary to refill the weir and effluent pipe with fresh water.
Another proposed decanting apparatus is disclosed in U.S. Pat. No. 5,358,644 to Dennis. The decanter disclosed in the Dennis patent includes a decanter receptacle, which has a top opening surrounded by a decanter rim, a decant pipe, and a float overlying the decanter receptacle in which the decanter rim is spaced below the float. In this apparatus, the seal between the top of the weir and the bottom of the float is not always sufficient to prevent suspended solids from entering the decant pipe, resulting in the discharge of water with suspended solids.
U.S. Pat. No. 5,421,995 to Norcross discloses a sludge level detector for floating decanters. The apparatus disclosed by Norcross includes a floating decanter assembly with a sludge detector light source and light sensor pair in a spaced-apart relation on a sludge layer detector frame, which is suspended below the floating decanter assembly. This apparatus provides some improvement over the prior art, however, its usefulness declines as the detector located at the bottom of the decanter assembly approaches the top of the sludge blanket.
Many floating decanter devices utilize decanting weirs or orifices that remain underwater throughout all sequences (even when not decanting), and they must incorporate various sealing arrangements in an attempt to prohibit mixed liquor suspended solids from entering the decanter. One such type of device uses a linear actuator to raise the weir during non-decant sequences and presses the weir lip against a gasket located on the bottom of the float. Since this seal will potentially leak, they must also close a motorized decant valve. A coil spring is required to provide the desired closing pressure force of 1,500 pounds. Another type of device utilizes spring loaded flapper valves for sealing the orifice openings and also close the motorized decant valve. Both of these devices have the decant entry (weir lip or an orifice opening) in the submerged position during non-decant sequences. In the event that the decant valve should leak, fail, or the sealing mechanism wear out, untreated wastewater would be discharged into the effluent pipework.
There is a demonstrated need for a floating decanter apparatus that can deliver supernatant without initial contamination of solids and can discharge water free of suspended solids through an automated operation, independent of the distance of the decanter from the top of the sludge blanket.